When you use the Replace-by-Fee (RBF) feature in your wallet, it creates a new transaction. BitPay invoices do not accept more than one transaction.
To ensure the payment you send is exactly the right amount and has a sufficient fee, BitPay now requires payments be sent with a Payment Protocol enabled wallet. Here’s our guide for paying a BitPay invoice using Payment Protocol.
When paying a BitPay invoice, you can increase the likelihood of timely confirmation by using a bitcoin wallet that dynamically calculates the miner fee to include in the transaction. We recommend the BitPay App. In the BitPay Wallet's settings, you can set the network fee to 'Priority'.
You can also check your wallet to see if you have a lot of small inputs which would make your outgoing transaction require higher fees. To resolve this issue, you can move all your funds to a single wallet and combine funds into a single input. For more information on how to consolidate funds to reduce miner fees, please check out the following wiki.
When using a BitPay Wallet, you can find out how many inputs the wallet has by selecting the desired wallet, then clicking on the preferences icon in the top right. Next, scroll down and tap Addresses, and scroll down to Total wallet inputs. You can also review the addresses with balances and select one use to consolidate. Use Send Max to move all your funds into a single input.
If using the Electrum Wallet, ensure that the RBF option is disabled by navigating to Electrum preferences and uncheck Use Replace-By-Fee option.